Non-invasive diagnostic transmission line testing

ABSTRACT

A probing signal transmitted on a twisted pair telephone line in a DSL system is reflected and received at a DSL device. An estimate of one of a DSL data transmission signal or DSL synch symbol transmission signal is removed from the received probing signal to recover the reflected probing signal. The recovered reflected probing signal is processed to determine characteristics information of the twisted pair telephone line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of International Application No.PCT/US2010/034267, filed May 10, 2010, the entire contents of which arehereby incorporated by reference herein.

FIELD OF THE DISCLOSURE

This disclosure relates generally to communications networks and/orsystems and, more particularly, to methods and apparatus to perform linetesting.

BACKGROUND

Digital subscriber line (DSL) technology is commonly utilized to provideinternet-related services to subscribers, such as, for example, homesand/or businesses (also referred to herein as users and/or customers).DSL technology enables customers to use telephone lines (e.g., ordinarytwisted-pair copper telephone lines used to provide Plain Old TelephoneSystem (POTS) services) to connect the customers to, for example, a highdata rate broadband Internet network, broadband service and/or broadbandcontent.

A service provider of a DSL service can use information such as looplength, cable gauge(s), presence of bridged tap(s), location of bridgedtap(s), lengths of bridged tap(s), noise on the line, shorts, opens,etc. for trouble detection, trouble isolation and/or trouble prevention.Alternatively or additionally, it may be useful to have similarinformation concerning the telephone line before DSL service is offered,sold and/or provisioned to a potential DSL subscriber, from the serviceprovider's location to the subscriber's location. Information such asthat mentioned above is measured for the telephone line between theservice provider's location and the subscriber's location. Existing linetesting methods would disrupt the operation of the DSL line. For exampleprior art Single Ended line (SELT) testing methods are performed bysending a probe signal on the DSL lines, and measuring the reflection inorder to estimate line characteristics, such as Return Loss or LineImpedance. This probing is done by disabling the operation of the DSLconnection and performing the SELT testing. This would disrupt the DSLline operation, which would cause interruption of the DSL service to thecustomers. Therefore SELT testings are done typically during newcustomer signups, or when the DSL line has a problem. This would avoidcausing service disruptions, however it prevents the service providerfrom having an up-to-date information on the status of the line.

Embodiments of the present invention overcome the above problems. Thepresent invention provides methods and systems for performing DSL linetesting, without disrupting the operation of the DSL line connectionthat is being tested. Therefore, it enables service providers to be ableto probe the DSL line at any time, in order to have up-to-dateinformation on the status of the DSL lines.

A line testing device (i.e. tester), operative to be located at acustomer premises, causes the transmission of a probing signal into atleast one telephone line. The tester computes a parameter thatrepresents a characteristic of at least one telephone line based upon atleast a measured reflected probing signal. Using any of a variety ofmethod(s), technique(s) and/or algorithm(s), the testers compute thecharacterizing parameter(s) from the measured reflected probing signal.For example, with knowledge of what probing signal was transmitted, andgiven a received and/or measured reflected signal, a tester can, forexample, compute an echo path response, detect the presence of a bridgedtap, characterize a detected bridged tap, estimate a loop attenuation,and/or determine any suitable telephone line characteristic. In somecircumstances, the measured reflected signals at the customer end of atelephone line will include a much greater level of detail about acustomer premises environment and/or the telephone line than would beavailable from reflected signals at the other (e.g., CO or RT) end.Therefore, the tester provides an enhanced level of detailed diagnosticsby performing one or more line test(s) from the customer premises 106.

While the following disclosure references the example digital subscriberline (DSL) system and/or the example of FIGS. 1-7, the methods andapparatus described herein may be used to characterize telephone linesfor any variety, any size and/or any topology of DSL system. Forexample, a DSL system may include more than one DSL access multiplexer(DSLAM) located in more than one location and may include any number oftelephone lines, DSL maintenance devices, testers, DSL modems and/ortesters. Also, for example, at customer premises, a plurality of modemscould terminate a plurality of telephone lines and share a single or aplurality of testers, data analyzers and/or computers. Additionally,although for purpose of explanation, the following disclosure refers toexample systems, devices and/or networks illustrated in FIG. 1A, anyadditional and/or alternative variety and/or number of communicationsystems, devices and/or network(s) may be used to implement a DSLcommunication system and/or provide DSL communication services inaccordance with the teachings disclosed herein. For example, thedifferent functions collectively allocated among a DSL managementcenter, a DSL access multiplexer (DSLAM), a DSL modem, a tester,computer, and/or a data analyzer as described below can be reallocatedin any desired manner.

As used herein, the terms “user”, “subscriber” and/or “customer” referto a person, business and/or organization to which communicationservices and/or equipment are and/or may potentially be provided by anyof a variety of service provider(s). Further, the term “customerpremises” refers to the location to which communication services arebeing provided by a service provider. For an example public switchedtelephone network (PSTN) used to provide DSL services, customer premisesare located at, near and/or are associated with the network termination(NT) side of the telephone lines. Example customer premises include aresidence or an office building.

As used herein, the term “operative” may describe an apparatus capableof an operation and/or actually in operation. For example, an apparatusoperable to perform some function describes a device turned off yet iscapable of performing an operation, by virtue of programming or hardwarefor example, and/or a device turned on and performing the operation. Theterm “signal” typically refers to an analog signal, the term “data”typically refers to digital data and the term “information” may refer toeither an analog signal and/or a digital signal although other meaningsmay be inferred from the context of the usage of these terms.

As used herein, the term “service provider” refers to any of a varietyof entities that provide, sell, provision, troubleshoot and/or maintaincommunication services and/or communication equipment. Example serviceproviders include a telephone operating company, a cable operatingcompany, a wireless operating company, an internet service provider, orany service that may independently or in conjunction with a DSL serviceprovider offer services that diagnose or improve the DSL service.

As used herein, the term “subscriber equipment” refers to any equipmentlocated at and/or in a customer premises for use in providing at leastone subscriber service. The subscriber equipment may or may not bepotentially available for additional purposes. While subscriberequipment is located at and/or in a customer premises, such equipmentmay be located on either side and/or both sides of a NT and/or any othernetwork ownership demarcation. Subscriber equipment may be owned,rented, borrowed and/or leased by a subscriber. Subscriber equipment maybe owned and entirely controlled by the service provider. For example,subscriber equipment could be owned by a service provider and thesubscriber only plugs into a connector and has no other access and/orinteraction with the device. Subscriber equipment is generally availableto and/or accessible by the subscriber and may be acquired and/orobtained by the subscriber via any of a variety of sources including,but not limited to, a retailer, a service provider, and/or an employer.Example subscriber equipment includes a personal computer (PC), aset-top box (STB), a residential gateway and/or a DSL modem located atand/or in a subscriber's residence by which the subscriber receivesand/or utilizes a DSL service and/or Internet services.

Additionally, as used herein, the term “DSL” refers to any of a varietyand/or variant of DSL technology such as, for example, Asymmetric DSL(ADSL), High-speed DSL (HDSL), Symmetric DSL (SDSL), and/or Veryhigh-speed DSL (VDSL). Such DSL technologies are commonly implemented inaccordance with an applicable standard such as, for example, theInternational Telecommunications Union (ITU) standard G.992.1 (a.k.a.G.dmt) for ADSL modems, the International Telecommunications Union (ITU)standard G.992.3 (a.k.a. G.dmt.bis, or G.adsl2) for ADSL2 modems, theInternational Telecommunications Union (ITU) standard G.992.5 (a.k.a.G.adsl2plus) for ADSL2+ modems, the International TelecommunicationsUnion (ITU) standard G.993.1 (a.k.a. G.vdsl) for VDSL modems, theInternational Telecommunications Union (ITU) standard G.993.2 for VDSL2modems, the International Telecommunications Union (ITU) standardG.994.1 (G.hs) for modems implementing handshake, and/or the ITU G.997.1(a.k.a. G.ploam) standard for management of DSL modems.

In the interest of brevity and clarity, throughout the followingdisclosure references will be made to connecting a DSL modem and/or aDSL communication service to a customer. However, while the followingdisclosure is made with respect to example digital subscriber line (DSL)equipment, DSL services, DSL systems and/or the use of ordinarytwisted-pair copper telephone lines for distribution of DSL services, itshould be understood that the disclosed methods and apparatus tocharacterize and/or test a transmission medium for communication systemsdisclosed herein are applicable to many other types and/or variety ofcommunication equipment, services, technologies and/or systems. Forexample, other types of systems include wireless distribution systems,wired or cable distribution systems, coaxial cable distribution systems,Ultra High Frequency (UHF)/Very High Frequency (VHF) radio frequencysystems, satellite or other extra-terrestrial systems, cellulardistribution systems, power-line broadcast systems and/or fiber opticnetworks. Additionally, combinations of these devices, systems and/ornetworks may also be used. For example, a combination of twisted-pairand coaxial cable connected by a balun, or any otherphysical-channel-continuing combination such as an analog fiber tocopper connection with linear optical-to-electrical connection at anoptical network unit (ONU) may be used.

It will be readily apparent to persons of ordinary skill in the art thatconnecting a DSL modem and/or tester to a customer involves, forexample, communicatively connecting the DSL modem and/or tester operatedby a communications company to a telephone line (i.e., a subscriberline) that is communicatively connected to a second DSL modem and/ortester located at and/or in a customer premises (e.g., a home and/orplace of business owned, leased or otherwise occupied and/or utilized bythe customer). The second DSL modem and/or tester may be furthercommunicatively connected to another communication and/or computingdevice (e.g., a personal computer) that the customer operates to accessa service (e.g., Internet access) via the first and second DSL modemsand/or tester, the telephone line and the communications company.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of an example system diagram.

FIG. 1B is a schematic illustration of an example system diagram.

FIG. 2 is a schematic illustration of an example system diagram.

FIG. 3 is a schematic illustration of an example system diagramperforming line testing.

FIG. 4 is a schematic illustration of an example method to perform linetesting.

FIG. 5 is a schematic illustration of an example method to perform linetesting.

FIG. 6 is a schematic illustration of an example method to perform linetesting.

FIG. 7 is a schematic illustration of an example method to perform linetesting.

SUMMARY

In one embodiment of the present invention a probing signal is sent onthe DSL line from the line test device, and the reflected signal isreceived by the line test device. FIG. 2 shows an example diagram of theembodiment.

The transmitted probing signal could be transmitted on the downstreamfrequencies or on the upstream frequencies. The transmitted probingsignal could also be transmitted in the upstream direction or indownstream direction. In one embodiment, for example the transmittedprobing signal could be transmitted on the downstream frequencies in theupstream direction. Therefore, the probing signal will not interferewith the upstream transmission, because the frequencies in the upstreamand downstream bands do not overlap. Such transmission could happen atthe CPE side. In another embodiment, for example the transmitted probingsignal could be transmitted on the upstream frequencies in thedownstream direction. Therefore, the probing signal will not interferewith the downstream transmission, because the frequencies in theupstream and downstream bands do not overlap. Such transmission couldhappen at the CO side.

The probing signal could be transmitted during the data transmissionperiod or during the Sync Symbol period. The Sync Symbol period is whenthe Sync Symbols are transmitted. Sync Symbols are transmitted toprovide synchronization support to the DSL communications.

In one embodiment of the present invention, the probing signal's poweris kept at a low Signal-to-Noise Ratio (SNR) level, in order to minimizethe interference between the probing signal and the DSL signal itself,whether during the Sync Symbol or data transmission periods. The probingsignal power level could be set as low as or smaller than noise power,resulting in a probing signal with negative SNR. Therefore the probingsignal would be constituted as noise by the signal receiver.

In another embodiment, the probe signal power can be larger than noisepower. The positive SNR could be utilized to recover the Probe signal inthe presence of residual noise after cancellation. Moreover, the Data orsynch symbol signal to noise ratio is large enough such that the probesignal would not interfere with proper decoding and reception of theSync symbol or Data symbols. An example would be a Data or Sync SymbolSNR of 50 dB, with probe signal SNR of 10 dB. The Data or Sync symbolhas a gain of 40 dB over the probe signal.

In one embodiment, the probing signal is recovered by cancelling theSync symbol or Data symbols from the received signal. In thisembodiment, first the Sync symbol or the data symbols are recovered.Then the estimated (recovered) Sync Symbol or data symbol are cancelled(removed) from the received signal. The Sync. symbol estimation is donevia the techniques, methods and systems disclosed in PCT Patentapplication no. PCT/US2009/036076, filed Mar. 4, 2009, titled “DSL NoiseCancellation Using Sub-Optimal Noise Reference Signals.” In case ofcancelling data symbols, the estimated data would be provided by the DSLreceiver at the DSL modem (i.e. DSL modems 102A-102C). In particular,when the testing device (i.e. 101B) and a DSL modem (i.e. 102B) areintegrated, the estimated DSL data would be readily available by the DSLreceiver. The recovered probing signal is further processed to obtainline characteristics information such as return loss and impedance.

In another embodiment the received probing signal is further processedto remove noise, either before the above cancellation step orafterwards. One technique for removing noise is averaging. For theaveraging process, sufficient samples of the received signal arecollected, and then the samples are averaged. The averaging processremoves the noise from the received signal.

FIG. 3 shows another embodiment of the present invention. In thisembodiment the probing signal may have a positive SNR. Therefore theprobing signal may be stronger than the received noise.

In order for the probing signal not to interfere with the DSL signalreception, the received probing signal is estimated using an adaptivefilter and is cancelled out from the received signal. The estimationprocess also provides an estimate of the reflected probing signal thatcould be processed to obtain line characteristics information such asreturn loss and impedance.

In this embodiment, the probing signal is processed by an adaptivefilter, and then cancelled from the received signal. The usage of theadaptive filter serves two objectives. One objective is to reproduce thereflected probing signal, which could be processed to obtain linecharacteristics information such as return loss and impedance.Alternatively, the coefficients of the adaptive filter could be used toobtain line characteristics. Another objective is to cancel out thereflected probing signal from the received signal, so that the receivercan process the received DSL signal, without any residue of thereflected probing signal, which might impair processing in the receiver.In this embodiment, the assumption is that reflected probing signalcould have positive SNR, therefore the signal might have large enoughmagnitude, which might impair the received signal processing.

The first objective is performed by the usage of an adaptive filter. Thetransmitted probe signal is used to drive the adaptive filter training,and the received signal is used to form the error signal for correctingthe filter adaptation. A number of adaptive filters, such as LMS, RLS,and variations of those could be used.

Once the adaptive filter coefficients have converged, and the trainingis complete, the adaptive filter canceller will be able to cancel outthe reflected probing signal from the received signal, achieving thesecond objective. Furthermore, the coefficients of the adaptive filterwould represent the probing signal reflection channel. This is becausethe input to the adaptive filter is the transmitted probing signal andthe output of the adaptive filter is the received reflected probingsignal. Therefore, the adaptive filter coefficients will represent theprobing signal reflection channel. Hence, the line characteristics, suchas return loss are readily obtained by knowing the coefficients of theadaptive filter representing the reflection channel.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A illustrates a DSL network involving one embodiment of theinvention. A bank of DSL modems, referred to as a DSLAM, (100) istypically located within a Central Office (105). Additionally, receivingDSL modems are each located at a Customer Premise (106). Telephone lines(103) carry the DSL signals from the DSLAM to the modems. Lines (103)may optionally provide voice band telephone signaling as well. Telephonelines 103 each typically comprises a twisted wire pair. Additionaltelephone lines (104) provide DSL communication to other customers.

Tester device (101B, 101C) represents one embodiment of the inventionthat is located within the customer premises. Tester devices (101A-C)perform line testing on the line (103), without disrupting the operationof DSL modems (102A-C). Other embodiments of the invention includeintegration of Tester device (101B) into a modem (102B), or integrationinto devices located outside of the customer premise (101A). The tester(101A, 101B, 101C), causes the transmission of a probing signal into atleast one telephone line 103. The tester computes a parameter thatrepresents a characteristic of the at least one telephone line 103 basedupon at least a measured reflection of the probing signal.

FIG. 1A illustrates an example DSL system that measures, computes and/orotherwise determines any number and/or any of a variety of parameter(s)that characterize, describe and/or indicate the state of ordinarytwisted-pair copper telephone line(s) that are being used and/or may beattempted to be used to provide DSL service(s) to the customer(s). Threesuch telephone lines are shown in FIG. 1A with reference numerals 103A,103B, and 103C. In the example system of FIG. 1A, the characterizingparameter(s) are measured, computed and/or otherwise determined basedupon one or more signals optionally transmitted from the customerpremises 106 and/or one or more signals received and/or measured at thecustomer premises 106. However, a DSLAM 100 may or may not terminate thetelephone lines 103A-C at other ends of the connections. Examplecharacterizing parameters include, but are not limited to loop length,segment length(s), cable gauge(s), bridged-tap presence, bridged-taplocation(s), bridged-tap length(s), bridged-tap gauge(s), open faults,short faults, cross faults, bad splice/connection, noise, excessivenoise, data rate, signal-to-noise ratio(s), loop impedance, loopmake-up, and/or loop attenuation/return loss. Alternatively oradditionally, raw data collected by receiving and/or measuring signal(s)from a telephone line 103A-C may be instead forwarded to ageographically separate device to compute these or other parameters(110). Such raw data may include digitized responses to pulse(s)launched by a line tester device 101A-C into the telephone line 103A-C,measurements of noise with no signals launched, and/or direct impedancemeasurements. As discussed below, the determination and/or computationof the characterizing parameter(s) based on the signals received and/ormeasured at the customer premises 106 may be implemented at the customerpremises 106 and/or at a geographically separate device (110).

To provide DSL services to the customer(s) via the example telephonelines 103A-C, the example system of FIG. 1A includes any variety ofDSLAM 100. The example DSLAM 100 of FIG. 1A implements, among otherthings, any of a variety and/or number of DSL modems (not shown). TheDSLAM 100 may be located in a central office (CO) and/or a remoteterminal (RT). Persons of ordinary skill in the art will appreciatethat, like other components described in the examples described herein,the DSLAM 100 need not be present.

To monitor, measure and/or record current and/or historical DSLperformance characteristics for DSL communications occurring between theexample DSLAM 100 and a plurality of subscriber DSL modems (three ofwhich are shown with reference numerals 101A, 101B and 101C), theexample DSL system of FIG. 1A includes a Management Center 110. TheManagement Center 110 may be part of, implemented by and/or performed byany or all of the following: a Spectrum Management Center (SMC), aDynamic Spectrum Management Center (DSM Center), a DSL Optimizer (DSLO),a DSL Management center, a DSL Operations Center, an Operations SupportSystem (OSS), an Element Management System (EMS), a Network ManagementSystem (NMS), other transmission or management network elements, and/orthe example DSLAM 100. As described below, the DSL example ManagementCenter 110 may request, receive, compute and/or otherwise obtain anynumber and/or any of a variety of parameters that characterize thetelephone line(s) and that are used to provide and/or may potentially beused to provide DSL services (e.g., the example telephone lines 103A-C).In the illustrated example, the telephone-line-characterizingparameter(s) and/or the performance characteristic(s) are stored in theexample Management Center 110 using any of a variety of datastructure(s), data table(s), data array(s), etc. Using any of a varietyof method(s), technique(s) and/or algorithm(s), a service provider or athird party, may use the telephone-line-characterizing parameter(s)and/or the performance characteristic(s), for example, to offer, selland/or provision new DSL services, and/or to maintain, monitor and/ordiagnose existing DSL services.

To measure signals from which the telephone-line-characterizingparameter(s) may be determined, the example system of FIG. 1A includestesters, which could be testers, at customer premises 106. Three exampletesters 101A, 101B, and 101C are shown in FIG. 1A. The example testers101A-C of FIG. 1A transmit any of a variety of line probing signalsand/or receive and/or measure any of a variety of reflected line probingsignals, crosstalk line probing signals and/or noise signals. Exampleprobing signals include pulse and/or step time domain reflectometry(TDR) signals, spread spectrum signals, nominal modem transmissionsignals (e.g., a multi-carrier signal of an ADSL modem), chirp signals,impulse trains, single impulse, etc. To measure noise conditions, a lineprobing signal may be a zero-voltage, quiet, null and/or all zerossignal such that, effectively, no signal is transmitted into a telephoneline being tested and/or characterized.

An example implementation of the example testers is discussed in PCTPatent application no. PCT/US2009/036076, filed Mar. 4, 2009, titled“DSL Noise Cancellation Using Sub-Optimal Noise Reference Signals” andU.S. patent application Ser. No. 12/226,939, filed Apr. 27, 2007, titled“Methods and Apparatus to Perform Line Testing at Customer Premises.”

Using any of a variety of method(s), technique(s) and/or algorithm(s),the example testers 101A-C estimate, determine and/or compute thecharacterizing parameter(s) from the received and/or measured signals.For example, with knowledge of what probing signal was transmitted, andgiven a received and/or measured reflected signal, a tester can, forexample, compute an echo path response, detect the presence of a bridgedtap, characterize a detected bridged tap, estimate a loop attenuation,etc. In some circumstances, the measured reflected signals at thecustomer end of a telephone line will include a much greater level ofdetail about a customer premises environment and/or the telephone linethan would be available from reflected signals at the other (e.g., CO orRT) end. Therefore, the illustrated example seeks to obtain thisenhanced level of detail by performing one or more line test(s) from thecustomer premises 106.

The example testers 101A-C of FIG. 1A may be implemented by any of avariety of computing devices such as, for example, a) a subscriber's PC,b) stand alone tester, and/or c) a DSL modem or d) a subscriber'sset-top box. For example, a PC implementing a tester 101B may beconnected to an Internet network and/or service via, for example, theDSL modem 102B. In such an example, the PC/tester 101B is used toreceive and/or utilize, for example, Internet, audio, video, email,messaging, television, and/or data services via the subscriber's DSLservice. In such an example, the PC 101B is connected to the Internetvia the DSL modem 102B, the telephone line 103B and the DSLAM 100.Accordion to one embodiment, the DSL modem 102B may be communicativelycoupled to the example PC 101B and/or be implemented by and/or withinthe example PC 1018.

The example testers 101A-C of FIG. 1A may execute machine accessibleinstructions to determine and/or compute thetelephone-line-characterizing parameter(s) from signal(s) receivedand/or measured by the corresponding example testers 101A-C. In theexample system of FIG. 1A, such machine accessible instructions may be(a) loaded into a tester via a compact disc (CD) or other non-volatilestorage (e.g., a digital versatile disc (DVD)) mailed and/or providedby, for example, a service provider; (b) downloaded to the tester 101A,1018, and 101C from an Internet site (e.g., a download server thatprovides machine accessible instructions provided by the ManagementCenter 110), and/or (c) loaded into the tester by, for example, theManagement Center 110. Any of a variety of network protocols such as,for example, hypertext transfer protocol (HTTP), file transfer protocol(FTP), and/or email protocols (e.g. SMTP) may be used to transfer themachine accessible instructions to the tester 101A, 1018, and 101C.

The characterizing parameter(s) determined and/or computed by theexample testers 101A-C are stored by and/or within the testers 101A-Cusing any of a variety of data structure(s), machine accessible file(s),and/or memory(ies). The example testers 101A-C of FIG. 1A provide thedetermined and/or computed characterizing parameter(s) to the ManagementCenter 110 via any of a variety of method(s), network(s) and/orprotocol(s). For example, if there is a DSL connection available and/oroperable between the DSL modems 102A-C and the DSLAM 100, the exampletesters 101A-C can provide the characterizing parameter(s) via the DSLservice using, for example, the exchange protocol defined in the ITUG.994.1 (a.k.a. G.hs) standard. Additionally or alternatively, thecharacterizing parameter(s) may be sent and/or provided to theManagement Center 110 via the Internet and/or a PSTN using, for example,a dial-up and/or voice-band modem communicatively coupled to, and/orimplemented by and/or within the tester 101A-C. Such a dial-up orvoice-band modem could operate over the voice band on the same loop asthe DSL service, or it could operate over a separate loop supportingPOTS service. A tester may, additionally or alternatively, provide thecharacterizing parameter(s) to the Management Center 110 via any of avariety of intermediary service(s) such as, for example, anAuto-Configuration Server (ACS) as defined in the DSL Forum documentTR-069. In the example of FIG. 1A, if a tester 101A, 101B, and/or 101Cis not currently communicatively coupled and/or couple-able to theexample Management Center 110, the characterizing parameter(s) may besent and/or provided via any of a variety of additional and/oralternative methods such as, for example, storing the characterizingparameter(s) on a CD or other non-volatile storage medium (e.g., a DVD)that can be sent and/or delivered to a service provider and then loadedinto the Management Center 110. Additionally or alternatively, a tester101A, 101B, and/or 101C can display the parameter(s) in, for example,the form of a condensed ASCII code using any of a variety of graphicaluser interfaces (GUIs) displayed for and/or presented to a person. Theexample person can in turn provide the parameter(s) to a technicianand/or customer service representative who in turn loads the providedparameter(s) into the Management Center 110. The person may be, forexample, a subscriber or technician.

As illustrated in FIG. 1A, the testers 101A-C may be implemented usingany of a variety of combinations. For example, the example tester 101Bis implemented by and/or within any of a variety of DSL modem(s) 102B,the example tester 101A is implemented as any of a variety ofstand-alone devices such as external to the CPE 106, the example tester101C is implemented by and/or within the example CPE 106 but separatefrom the modem 102C. Also, a single prober could be implemented byand/or within multiple DSL modems present at the customer premises 106.Persons of ordinary skill in the art will readily appreciate that thereare a multitude of other ways of implementing testers. For example, atester may be implemented by any of variety of residential gateways orSTBs.

The example testers 101A and 101C may be communicatively coupled totheir respective Modems 101A and 101C via any of a variety ofcommunication buses, backplanes, wired and/or wireless signals and/ortechnologies such as, a universal serial bus (USB), and/or a wiredand/or wireless connection in accordance with the Institute ofElectrical and Electronics Engineers (IEEE) 802.3x and/or 802.11xstandards. Additionally, a tester 101B may be implemented by and/orwithin a DSL modem tester 102B, using, for example, a peripheralcomponent interface (PCI) card.

In the example system of FIG. 1A, determination and/or computation ofthe parameter(s) that characterize a telephone line may be initiated,requested and/or provided in any of a variety of ways. For example, theexample Management Center 110 may send a request and/or command to atester 101A, 101B, and/or 101C requesting the transmission of probingsignals and/or requests reception and/or measurement of signals from thetester 101A, 101B, and/or 101C. The tester 101A, 101B, and/or 101C mayadditionally compute and/or determine the characterizing parameter(s)from the received and/or measured signals, and then return the same tothe Management Center 110 as discussed above. Additionally oralternatively, a DSL subscriber, technician, installer, etc. mayinitiate the process of transmitting probing signals, signalmeasurement, and/or characterizing parameter computation and/ordetermination via any of a variety of GUIs provided and/or displayed bya tester 101A, 101B, and/or 101C. Finally, the transmission of probingsignals may be initiated by a DSL modem operating in loop diagnosticmode. The tester itself may make regular or periodic attempts toidentify itself to a service provider maintenance device or to amanagement center through any of the above-mentioned electroniccommunication paths. Thus, its release of data need not necessarily beprompted by a service provider.

In the illustrated example of FIG. 1A, example testers 101A, 101B,and/or 101C implemented external to, or by and/or within a DSL modem,residential gateway, etc. have access to alternating current (AC) and/orbattery power, even if the DSL modem or residential gateway is in a lowpower state and/or is turned off. This allows a communicatively coupledtester 101A, 101B, and/or 101C to perform line testing, probing and/orsignal measuring independent of the state of the DSL modem orresidential gateway. Thus, line testing, probing and/or characterizingcan be performed by technicians, maintenance personal and/or customerservice representatives even if the DSL modem or residential gateway isturned off. In such circumstances, the sending of request(s) to thetester occurs via another existing and/or available connection betweenthe tester 101A, 101B, and/or 101C and the Internet and/or the PSTN,and/or via a user operating, for example, a GUI displayed and/orprovided by the tester 101A, 101B, and/or 101C.

While in the example of FIG. 1A, the example testers 101A-C are locatedat or near the customer premises 106, persons of ordinary skill in theart will readily appreciate that, additionally or alternatively, testers101A, 101B, and/or 101C may be implemented at a CO or RT, as shown inFIG. 1B. For example, a tester 101A, 101B, and/or 101C could beimplemented by and/or within the example DSLAM 100. As shown in FIG. 1B,in one example, the tester (e.g., tester 101A, 101B, and/or 101C)provides received and/or measured probing and/or noise signals to theremotely located Management Center 110. Further, while FIG. 1Aillustrates one tester 101A-C for each line or loop (103), persons ofordinary skill in the art will readily appreciate that a tester that is,for example, located at a CO may determine and/or compute characterizingparameter(s) for more than one telephone line using received and/ormeasured signals from more than one tester 101A-C located at CO 105.Further still, one or more testers may be implemented by (101C), within(101B) and/or in conjunction (101A) with a DSLAM to provide linetesting, probing and/or characterizing from the service provider's endof a telephone line.

In the illustrated example of FIGS. 1 and 1B, the example ManagementCenter 110 can also use the set of testers 101A-C to measure and/orcharacterize near-end and/or far-end cross-talk. For example, a firsttester at a first customer premises 106 (e.g., the example tester101A-C) can be configured to transmit a probing signal into a firsttelephone line (e.g., the telephone line 103A) while, at substantiallythe same time, a second tester at a second customer premises 106 (e.g.,the example tester 101B) suspends transmission of the line probingsignal, transmits a “quiet” signal, or no signal into a second telephoneline (e.g., the telephone line 103B). A signal then received andmeasured by the second tester 101B can be used to characterize so called“near-end crosstalk” from the first telephone line 103A associated withthe first tester 101A into the second telephone line 103B associatedwith the second tester 101B. If the second tester 101B is insteadlocated at the CO end 105 of the second telephone line 103B, then thesignal received and measured by the second tester 101B can be used tocharacterize so called “far-end crosstalk” from the first telephone line103A into the second telephone line 103B. The second tester 101B of theillustrated example need not send signals into the line to measuresignals from the line. Instead, the illustrated example second tester101B may collect and save samples from the line 103B at regular and/orirregular intervals to assess noise at the customer premises 106 and/orCO 105. Such samples can be stored in the second tester 101B and then beprovided when the second tester 101B is interrogated by the tester 101B,forwarded at scheduled times, forwarded upon the occurrence ofpredetermined events (e.g. storage of a predetermined amount of data),and/or at other periodic and/or aperiodic times.

FIG. 2 shows one embodiment of the present invention where a probingsignal (204) is sent on the DSL line from the line test device (214),and the reflected signal (216) is received by the line test device(214). The hybrids (210) and (212) connect the test device to the CO(200) and CPE (202) sides respectively.

In DSL standards that employ frequency-division duplexing (FDD) ofupstream and downstream data transmission, the transmitted probingsignal (204) could be transmitted on the frequencies used for downstreamtransmission or on the frequencies used for upstream data transmission.The transmitted probing signal (204) could also be transmitted in theupstream direction (208) or in downstream direction (206), i.e. eitherat the CPE side or the CO side. In FIG. 2, the example embodiment showsthe probe signal being transmitted in the upstream direction (at the CPEside). In this example the transmitted probing signal is transmitted onthe frequencies used for downstream data transmission by the DSL link.Therefore, the probing signal will not interfere with the upstreamtransmission, because the frequencies in the upstream and downstreambands do not overlap. However, such a probing signal might have thepotential to interfere with downstream transmission because it occurs inthe same frequency bands used by the DSL modem for downstream datatransmission. This phenomenon is referred to as “in-band interference”.In another embodiment, the transmitted probing signal could betransmitted on the upstream frequencies in the downstream direction (atthe CO side). Therefore, the probing signal will not interfere with thedownstream transmission, because the frequencies in the upstream anddownstream bands do not overlap. However, such a probing signal has thepotential to interfere with upstream transmission because it occurs inthe same frequency bands as used by the DSL modem for upstream datatransmission. This phenomenon is also referred to as “in-bandinterference”.

Furthermore, the probing signal could be transmitted during the datatransmission period or during a Sync Symbol period. A Sync Symbol periodis the duration of time during which a Sync Symbol is transmitted by aDSL modem. One of the Sync Symbol transmission periods may be during themodem initialization period. Additionally, Sync Symbols may betransmitted to provide synchronization support to the DSL communicationsduring normal modem operation (SHOWTIME).

In one embodiment of the present invention, the probing signal's poweris kept at a low SNR level, in order to minimize the interferencebetween the probing signal and the DSL signal itself, whether during theSync Symbol or data transmission periods. The probing signal power levelcould be set as low as or smaller than noise power, resulting in aprobing signal with negative SNR. Therefore the probing signal would beconstituted as noise by the signal receiver. FIG. 4 shows one embodimentof the present invention where the probing signal is recovered bycancelling the Sync symbol or Data symbols from the received signal. Inthis embodiment, first the probe signal is transmitted on the line, step(400). The received signal is then received (step 402) by the testdevice (214).

The received signal can be represented as the following:y(k)=Sync(k)+N(k)+R _(probe)(k);when test probe signal is sent during Sync symboly(k)=Data(k)+N(k)+R _(probe)(k);when test probe signal is sent during data symbol whereR _(probe)(k)=reflection_channel(k)*T _(probe)(k)where y(k) represents the received signal at time sample k, Sync(k)represents the received Sync. symbol at time sample k, Data(k)represents the received data symbol at time sample k, N(k) representsthe noise sample at time sample k, R_(probe)(k) represents the reflectedprobe signal at time sample k, T_(robe)(k) represents the transmittedprobe signal at time sample k, and Reflection_Channel(k) represents theimpulse response of the reflected channel, and * represents theconvolution function.

The goal is to recover the R_(probe)(k) signal, which could be processedto obtain line characteristics information such as return loss andimpedance.

In one embodiment of the present invention, the noise is removed fromthe received signal y(k) before removing the Sync. or Data symbols, step(404). In the case of removing only the Sync symbol, the probe signal istransmitted during the sync period. And in the case of removing the Datasymbol, the probe signal is transmitted during the data period. Onemethod for removing noise is averaging. For the averaging process,sufficient samples of the received signal are collected, and then thesamples are averaged. The averaging process removes the noise from thereceived signal. The Sync. or the Data signal is then cancelled bysubtracting an estimate of the Sync symbol or the Data signal from thenoise-removed y(k), step (406). The Sync symbol or the Data symbols areestimated independently, step (408). Then the estimated (recovered) SyncSymbol or Data symbol are cancelled (removed) from the received signal,step 406. The Sync. symbol estimation is done via the techniques,methods and systems disclosed in PCT Patent application no.PCT/US2009/036076, filed Mar. 4, 2009, titled “DSL Noise CancellationUsing Sub-Optimal Noise Reference Signals.” In case of cancelling datasymbols, the estimated data would be provided by the DSL receiver at theDSL modem (such as DSL modems 102A-102C). In particular, when thetesting device (i.e. 101B) and a DSL modem (i.e. 102B) are integrated,the estimated DSL data would be readily available by the DSL receiver.The recovered probing signal is further processed to obtain linecharacteristics information such as return loss and impedance (412).

The process could be described as following for when test probe signalis sent during Data symbol:

-   -   Receiving y(k):        y(k)=Data(k)+N(k)+R _(probe)(k)    -   Remove noise:        remove_noise(y(k))=Data(k)+R _(probe)(k)    -   Estimate Data(k)        estimated_Data(k)=estimate(D(k))    -   Cancelling/removing Data(k):        remove_noise(y(k))−estimated_Data(k)=R _(probe)(k)

The process could be described as following for when test probe signalis sent during Sync. symbol:

-   -   Receiving y(k):        y(k)=Sync(k)+N(k)+R _(probe)(k)    -   Removing noise from y(k):        remove_noise(y(k))=Sync(k)+R _(probe)(k)    -   Estimate Sync(k)        estimated_Sync(k)=estimate(Sync(k))    -   Cancelling/removing Sync(k):        remove_noise(y(k))−estimated_Sync(k)=R _(probe)(k)

The recovered received probe signal R_(probe) (k) is then analyzed tomeasure and calculate the line characteristics information, such asreturn loss and impedance.

FIG. 3 shows another embodiment of the present invention. However, inthis embodiment, the probing signal may have a positive SNR. The signalmay be stronger than the received noise. In order to avoid having thepositive SNR probing signal create “in-band interference”, the reflectedprobing signal is estimated using an adaptive filter (318), withcoefficients (W) representing the reflection channel, and the reflectedprobing signal is removed from the received signal (306) before beingpassed to the hybrid (312). The coefficients (W) may be processed toobtain line characteristics information such as return loss andimpedance.

The probing signal (304) is sent on the DSL line from the line testdevice (314), and the reflected signal (316) is received by the linetest device (314). The hybrids (310) and (312) connect the test deviceto the CO (300) and CPE (302) sides respectively.

The transmitted probing signal (304) could be transmitted on thedownstream frequencies or on the upstream frequencies. The transmittedprobing signal (304) could also be transmitted in the upstream direction(308) or in downstream direction (306). In FIG. 3, the exampleembodiment shows the probe signal being transmitted in the upstreamdirection. In this example the transmitted probing signal is transmittedon the downstream frequencies in the upstream direction. Therefore, theprobing signal will not interfere with the upstream transmission,because the frequencies in the upstream and downstream bands do notoverlap. Such transmission could happen at the CPE side. In anotherembodiment, the transmitted probing signal could be transmitted on theupstream frequencies in the downstream direction. Therefore, the probingsignal will not interfere with the downstream transmission, because thefrequencies in the upstream and downstream bands do not overlap. Suchtransmission could happen at the CO side. In another embodiment, theprobing signal is transmitted during the Sync Symbol period or the Dataperiod. In this embodiment, the probing signal is recovered by anadaptive filter (318), with coefficients (W) representing the reflectionchannel, and then cancelled from the received signal.

FIG. 5, shows a functional diagram of the above embodiment. In thisembodiment, first the probe signal is transmitted on the line, step(500). The received signal is then received (step 502) by the testdevice (214).

The received signal can be represented as the following:y(k)=Sync(k)+N(k)+R _(probe)(k);when test probe signal is sent during Sync symbol, ory(k)=Data(k)+N(k)+R _(probe)(k);when test probe signal is sent during Data symbol whereR _(probe)(k)=reflection_channel(k)*T _(probe)(k)where y(k) represents the received signal at time sample k, Sync(k)represents the received Sync symbol at time sample k, Data(k) representsthe received Data symbol at time sample k, N(k) represents the noisesample at time sample k, R_(probe)(k) represents the reflected probesignal at time sample k, T_(probe)(k) represents the transmitted probesignal at time sample k, and Reflection_Channel(k) represents theimpulse response of the reflected channel, and * represents theconvolution function.

The usage of the adaptive filter serves two objectives. One objective isto recover the R_(probe)(k) signal, which could be processed to obtainline characteristics information such as return loss and impedance.Another objective is to cancel out R_(probe)(k) from the receivedsignal, so that the DSL receiver at the DSL modem (i.e. DSL modems102A-102B) can process y(k), without any residue R_(probe)(k) whichmight impair the Sync. symbol or Data symbol processing in the receiver.In this embodiment, the assumption is that R_(probe)(k) could havepositive SNR, therefore the signal might have large enough magnitude,which might impair the received signal processing, during the synch.symbol or Data symbol processing time.

Both objectives may be achieved by the usage of an adaptive filter(506). The inputs to the adaptive filtering algorithm are thetransmitted probe signal (304) and the received signal (306). Theadaptive filtering algorithm attempts to find filter coefficients toremove the transmitted probe signal (304) from the received signal(306). The adaptive filtering algorithm used could be least mean-squares(LMS), regularized least mean-squares (RLS), time-domain processing,frequency-domain processing, or other such adaptive filtering algorithmsas would be known to those proficient in the art.

As the adaptive filter coefficients converge, the adaptive filtercanceller will be able to remove the reflected probing signal from thereceived downstream signal (504) with increasing efficacy such that theeffect of any “in-band interference” is transient. Furthermore, thecoefficients of the adaptive filter may themselves contain informationthat may be used to estimate the probing signal reflection channel(508). This is because the input to the adaptive filter is thetransmitted probing signal and the output of the adaptive filter is thereceived reflected probing signal. Therefore, the adaptive filtercoefficients will represent the probing signal reflection channel.

Line characteristics, such as return loss and impedance, may beestimated from the coefficients of the adaptive filter representing thereflection channel using algorithms and techniques including but notlimited to: matching the observed reflection channel to models ofreflection channels of measured or modeled loop configurations, matchingthe observed reflection channel to historical statistics computed fromthe reflection channel of the same or neighboring lines, comparing theobserved reflection channel to the reflection channel when other lineterminations, impairments, or alterations are present on the same line,and using Bayesian inference to combine reflection channel data withhistorical reflection channel data or other data sources. Such analysesmay be conducted at the Tester (101A/B/C) or at the Management Center(110), or by a combination of actions at each device. The results ofsuch analyses may be stored, transferred, or exported by or to theManagement Center (110).

One embodiment of the above process could be further described asfollowing. The received signal y(k) is utilized to train the adaptivefilter (506). The output of the adaptive filter is the estimatedreflected probing signal. The reflected probing signal is then cancelledby subtracting the estimate of the reflected probing from the receivedsignal y(k) (504).

-   -   Receiving y(k):        y(k)=Sync(k)+N(k)+R _(probe)(k);        when test probe signal is sent during Sync symbol        or        y(k)=Data(k)+N(k)+R _(probe)(k);        when test probe signal is sent during Data symbol-Adaptive        filtering of y(k):        R _(probe)(k)=adaptive_filter(y(k),T _(probe)(k))        Cancelling/removing R_(probe)(k):        y(k)−R _(probe)(k)=Sync(k)+N(k)

In another embodiment of the above process, the adaptive filtering canbe done during both Sync Symbol period and Data Symbol period. Theinitial adaptation can start during the Sync Symbol period, whendecoding is less sensitive to the input SNR of the adaptive filter, andfurther adaptation could be continued during Data Symbol period. FIG. 6shows another embodiment of the present invention that utilizes thetechnique of recovering the probe signal by cancelling the Sync symbolor Data symbols from the received signal. In this embodiment, first theprobe signal is transmitted on the line, step (600). The received signalis then received (step 602) by the test device (214).

The received signal can be represented as the following:y(k)=Sync(k)+N(k)+R _(probe)(k);when test probe signal is sent during Sync symboly(k)=Data(k)+N(k)+R _(probe)(k);when test probe signal is sent during data symbol whereR _(probe)(k)=reflection_channel(k)*T _(probe)(k)where y(k) represents the received signal at time sample k, Sync(k)represents the received Sync symbol at time sample k, Data(k) representsthe received data symbol at time sample k, N(k) represents the noisesample at time sample k, R_(probe)(k) represents the reflected probesignal at time sample k, T_(probe)(k) represents the transmitted probesignal at time sample k, and Reflection_Channel(k) represents theimpulse response of the reflected channel, and * represents theconvolution function.

The goal is to recover the R_(probe)(k) signal, which could be processedto obtain line characteristics information such as return loss andimpedance.

In this embodiment, the noise is removed from the received signal y(k)after removing the Sync. or Data symbols, step (604).

The Sync. or the Data signal is cancelled by subtracting an estimate ofthe Sync symbol or the Data signal from the received signal y(k), step(606). The Sync symbol or the Data symbols are estimated independently,step (608). Then the estimated (recovered) Sync Symbol or Data symbolare cancelled (removed) from the received signal, step (606). The Sync.symbol estimation is done via the techniques, methods and systemsdisclosed in PCT Patent application no. PCT/US2009/036076, filed Mar. 4,2009, titled “DSL Noise Cancellation Using Sub-Optimal Noise ReferenceSignals.” In case of cancelling data symbols, the estimated data wouldbe provided by the DSL receiver at the DSL modem (i.e. DSL modems102A-102C). In particular, when the testing device (i.e. 101B) and a DSLmodem (i.e. 102B) are integrated, the estimated DSL data would bereadily available by the DSL receiver. The recovered probing signal isfurther processed to obtain line characteristics information such asreturn loss and impedance.

The next step involves removing noise (604). One method for removingnoise is averaging. For the averaging process, sufficient samples of thereceived signal are collected, and then the samples are averaged. Theaveraging process removes the noise from the received signal.

The process could be described as following for when test probe signalis sent during Data symbol:

-   -   Receiving y(k):        y(k)=Data(k)+N(k)+R _(probe)(k)    -   Estimated Data(k)        estimated_Data(k)=estimate(Data(k))    -   Cancelling/removing Data(k):        y(k)−estimated_Data(k)=N(k)+R _(probe)(k)    -   Remove noise:        remove_noise[y(k)−estimated_Data(k)]=R _(probe)(k)

The process could be described as following for when test probe signalis sent during Sync. symbol:

-   -   Receiving y(k):        y(k)=Sync(k)+N(k)+R _(probe)(k)    -   Estimated Sync(k)        estimated_Sync(k)=estimate(Sync(k))    -   Cancelling/removing Sync(k):        y(k)−estimated_Sync(k)=R _(probe)(k)    -   Removing noise:        remove_noise[y(k)−estimated_Sync(k)]=R _(probe)(k)

The recovered received probe signal R_(probe)(k) is then analyzed tomeasure and calculate the line characteristics information, such asreturn loss and impedance.

FIG. 7 shows another embodiment of the present invention that utilizesthe technique of recovering the probe signal by cancelling the Syncsymbol or Data symbols from the received signal. In this embodiment,first the probe signal is transmitted on the line, step (700). Thereceived signal is then received (step 702) by the test device (214).

The received signal can be represented as the following:y(k)=Sync(k)+N(k)+R _(probe)(k);when test probe signal is sent during Sync symboly(k)=Data(k)+N(k)+R _(probe)(k);when test probe signal is sent during data symbol whereR _(probe)(k)=reflection_channel(k)where y(k) represents the received signal at time sample k, Sync(k)represents the received Sync symbol at time sample k, data(k) representsthe received data symbol at time sample k, N(k) represents the noisesample at time sample k, R_(probe)(k) represents the reflected probesignal at time sample k, T_(probe)(k) represents the transmitted probesignal at time sample k, and Reflection_Channel(k) represents theimpulse response of the reflected channel, and * represents theconvolution function.

The goal is to recover the R_(probe)(k) signal, which could be processedto obtain line characteristics information such as return loss andimpedance.

The Sync. or the Data signal is cancelled by subtracting an estimate ofthe Sync symbol or the Data signal from the received signal y(k), step(706). The Sync symbol or the Data symbols are estimated independently,step (708). Then the estimated (recovered) Sync Symbol or Data symbolare cancelled (removed) from the received signal, step (706). The Sync.symbol estimation is done via the techniques, methods and systemsdisclosed in PCT Patent application no. PCT/US2009/036076, filed Mar. 4,2009, titled “DSL Noise Cancellation Using Sub-Optimal Noise ReferenceSignals.” In case of cancelling data symbols, the estimated data wouldbe provided by the DSL receiver at the DSL modem (i.e. DSL modems102A-102C). In particular, when the testing device and a DSL modem areintegrated, the estimated DSL data would be readily available by the DSLreceiver. The recovered probing signal is further processed to obtainline characteristics information such as return loss and impedance.

The process could be described as following for when test probe signalis sent during Data symbol:

-   -   Receiving y(k):        y(k)=Data(k)+N(k)+R _(probe)(k)    -   Estimated Data(k)        estimated_Data(k)=estimate(D(k))    -   Cancelling/removing Data(k):        y(k)−estimated_Data(k)=N(k)+R _(probe)(k)

The process could be described as following for when test probe signalis sent during Sync. symbol:

-   -   Receiving y(k):        y(k)=Sync(k)+N(k)+R _(probe)(k)    -   Estimated Sync(k)        estimated_Sync(k)=estimate(Sync(k))    -   Cancelling/removing Sync(k):        y(k)−estimated_Sync(k)=N(k)+R _(probe)(k)

The recovered received probe signal R_(probe)(k) is then analyzed tomeasure and calculate the line characteristics information, such asreturn loss and impedance. In this embodiment, the probe signal powercan be larger than noise power. The positive SNR could be utilized torecover the Probe signal in the presence of residual noise aftercancellation. Moreover, the Data or synch symbol signal to noise ratiois large enough such that the probe signal would not interfere withproper decoding and reception of the Sync symbol or Data symbols. Anexample would be a Data or Sync Symbol SNR of 50 dB, with probe signalSNR of 10 dB. The Data or Sync symbol has a gain of 40 dB over the probesignal.

The cancellation techniques discussed in the disclosed embodiments couldlead to residual noise. In order to whiten the residual noise, thetransmitted probing signal could be randomized. Since the transmittedprobing signal timing, size and shape is known, the receiver can stillrecover the probing signal. The randomization causes the residualestimation or cancellation noise to become white.

Therefore the noise cancellation methods discussed in the variousembodiments could be used to cancel out the residual noise as well. Inparticular, averaging noise cancellation techniques would also cancelout the randomized residual noise. The randomization can be done todifferent characteristics of the transmitted probe signal, such asshape, timing, and power. One exemplary randomization would be thetransmission time jittering. The starting time of the probing signalcould be randomized with a known pattern. The randomization patternwould be known at the receiver therefore the received probing signalcould be recovered. However, as a result of the timing jitter, theresidual noise after cancelling Data or Sync Symbols would be randomizedand the resulting noise would resemble white noise. Therefore,subsequently, noise cancellation methods, such as averaging, couldcancel out the residual noise.

The invention claimed is:
 1. A method comprising: transmitting a probingsignal on the twisted pair telephone line from a Digital Subscriber Line(DSL) device coupled to the line, wherein the probing signal istransmitted in a frequency used for upstream data transmission ordownstream data transmission in a DSL system that employsfrequency-division duplexing of upstream and downstream datatransmission, in one of an upstream direction from a customer premiseslocation or a downstream direction from a central office location,during one of a DSL data transmission period or DSL sync symboltransmission period; receiving the probing signal reflected on thetwisted pair telephone line at the DSL device; removing an estimate ofone of a DSL data transmission signal or DSL synch symbol transmissionsignal from the received probing signal to recover the reflected probingsignal; and processing the recovered reflected probing signal todetermine characteristics information of the twisted pair telephoneline.
 2. The method of claim 1, wherein a power of the probing signal iskept at a signal to noise ratio that is substantially equal to or lessthan a noise power of a twisted pair telephone line during a period ofDSL data transmission or a DSL synch symbol transmission on the line. 3.The method of claim 1, wherein the probing signal is selected from agroup of signals consisting of a pulse signal, a step time domainreflectometry (TDR) signal, a spread spectrum signal, a nominal modemtransmission signal, a chirp signal, an impulse train signal, and asingle impulse signal.
 4. The method of claim 1, wherein receiving aprobing signal reflected on a twisted pair telephone line at a DSLdevice comprises receiving the reflected probing signal at a DSL deviceselected from a group of DSL devices consisting of a line testingdevice, a DSL modem, a DSLAM, a DSL subscriber's computing device, and aset-top box.
 5. The method of claim 1, further comprising removing noisefrom the received probing signal.
 6. The method of claim 5, wherein thenoise is removed from the received probing signal before removing theestimate of one of a data signal or synch symbol signal from thereceived probing signal to recover the reflected probing signal.
 7. Themethod of claim 5, wherein the noise is removed from the receivedprobing signal after removing the estimate of one of a data signal orsynch symbol signal from the received probing signal to recover thereflected probing signal.
 8. The method of claim 5, wherein removingnoise from the received probing signal comprises receiving a pluralityof samples of the probing signal and averaging the samples to remove thenoise from the received probing signal.
 9. The method of claim 1,further comprising obtaining the estimate of one of a data signal orsynch symbol signal for use in removing the estimate from the receivedprobing signal to recover the reflected probing signal.
 10. The methodof claim 9, wherein obtaining the estimate comprises receiving theestimate from a DSL receiver of a DSL modem.
 11. The method of claim 1,wherein to determine line characteristics information comprisescomputing a parameter that represents a characteristic of the twistedpair telephone line based on the recovered reflected probing signal, andwherein the characteristic is selected from a group of characteristicsconsisting of: a return loss, an impedance, an echo path response, adetection of a presence of a bridged tap, a characterization of thedetected bridged tap, and an estimate of loop attenuation.
 12. A DSLdevice comprising: an interface coupled to a twisted pair telephone lineto transmit a probing signal on the twisted pair telephone line and toreceive the probing signal reflected on the line, wherein the probingsignal is transmitted in a frequency used for upstream data transmissionor downstream data transmission in a DSL system that employsfrequency-division duplexing of upstream and downstream datatransmission, in one of an upstream direction from a customer premiseslocation or a downstream direction from a central office location,during one of a DSL data transmission period or DSL sync symboltransmission period; logic coupled to the interface to remove anestimate of one of a DSL data transmission signal or DSL synch symboltransmission signal from the received probing signal to recover thereflected probing signal; and processing logic coupled to the logic toprocess the recovered reflected probing signal to determinecharacteristics information of the twisted pair telephone line.
 13. TheDSL device of claim 12, wherein the probing signal is selected from agroup of signals consisting of a pulse signal, a step time domainreflectometry (TDR) signal, a spread spectrum signal, a nominal modemtransmission signal, a chirp signal, an impulse train signal, and asingle impulse signal.
 14. The DSL device of claim 12, wherein the DSLdevice is selected from a group of DSL devices consisting of a linetesting device, a DSL modem, a DSLAM, a DSL subscriber's computingdevice, and a set-top box.
 15. The DSL device of claim 12, furthercomprising logic to remove noise from the received probing signal eitherbefore removing the estimate of one of a data signal or synch symbolsignal from the received probing signal to recover the reflected probingsignal or after removing the estimate of one of a data signal or synchsymbol signal from the received probing signal to recover the reflectedprobing signal.
 16. The DSL device of claim 12, wherein the logiccoupled to the interface to remove an estimate of one of a DSL datatransmission signal or DSL synch symbol transmission signal from thereceived probing signal obtains the estimate from a DSL receiver of aDSL modem.